The present invention relates to modulation of data transmission, more specifically, to phase-Shift Keying (PSK).
Digital modulation converts digital data to analog symbols for physical transmission in digital communication, with digital demodulation as the reverse process. The modulation process, enabling the transmission involves switching (keying) the amplitude, frequency, or phase of a sinusoidal carrier in some fashion in accordance with the incoming digital data. Basic signaling schemes are amplitude-shift keying (ASK), frequency-shift keying (FSK), and phase-shift keying (PSK). Both PSK and FSK signals have a constant envelope, and because of this property, are impervious to amplitude nonlinearities, commonly encountered in the communication channels. PSK and FSK signals are preferred to ASK signals for data transmission over nonlinear channels.
Gaussian-prefiltered Minimum Shift Keying and π/4 (quarter pi) QPSK (quaternary phase shift keying) are common digital mobile modulation/demodulation methods in Europe and United Sates respectively, with π/4 Differential QPSK (DQPSK) adopted by IS-54 (TDMA CDMA), PACS (Low power) and PHS in the current market. Common PSK methods include Binary Phase Shift Keying (BPSK), QPSK and its variants.
FIG. 1 is a BPSK constellation diagram showing that each bit may be transmitted by varying the phase according to its value. Positions corresponding to phase=0 and phase=π on the BPSK constellation diagram differentiate the bit as logic high or logic low. FIG. 2 illustrates an example of six bits transmitted by the BPSK method, with a whole cycle of cosine wave corresponding to logic low (0), and a whole cycle of negative cosine wave corresponding to logic high (1). Note that there is an 180° (π) phase change whenever the bit value changes from 0 to 1 or 1 to 0, with a sharp phase change resulting in a spiky pulse in the time domain, requiring high transmission frequency and wider bandwidth, undesirable in the communication system.
FIG. 3 is a QPSK constellation diagram, wherein each symbol carries 2 bits of information, and is π/2 (90°) out of phase with its neighbors. FIG. 4 illustrates that the maximum phase change between consecutive symbols in QPSK is also n (180°), which creates the same bandwidth problem as BPSK.
π/4 QPSK is another modulation alternative which further divides the constellation diagram into eight directions, but the number of bits transmitted per symbol remains two. Since an additional bit differentiates the eight directions as either odd or even. FIG. 5 is a constellation diagram for π/4 QPSK, wherein the phases 0, π/4, π/2, 3π/4, π, −3π/4, −π/2, and −π/4 correspond to (0,0) even, (0,0) odd, (1,0) even, (1,0) odd, (1,1) even, (1,1) odd, (0,1) even, and (0,1) odd respectively. Pairs with the same bit information are grouped next to each other with a phase difference of π/4, with no two “odd” or two “even” symbols next to each other. The system transmits symbols alternating between even and odd, so that the largest possible phase difference between two consecutive symbols is reduced to 3π/4. The greater the phase difference in the time domain, the sharper the pulse at the transition of two symbols, thereby requiring a broader spectrum in the frequency domain. It is crucial to keep the bandwidth as narrow as possible in order to employ the limited spectrum resources efficiently. Communication quality degrades if the bandwidth required by the transmitted signal exceeds its assigned channel bandwidth, causing interference with other signals.